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A Framework for Conferencing with the Session Initiation Protocol (SIP)
RFC 4353

Network Working Group                                       J. Rosenberg
Request for Comments: 4353                                 Cisco Systems
Category: Informational                                    February 2006

                 A Framework for Conferencing with the
                   Session Initiation Protocol (SIP)

Status of This Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   The Session Initiation Protocol (SIP) supports the initiation,
   modification, and termination of media sessions between user agents.
   These sessions are managed by SIP dialogs, which represent a SIP
   relationship between a pair of user agents.  Because dialogs are
   between pairs of user agents, SIP's usage for two-party
   communications (such as a phone call), is obvious.  Communications
   sessions with multiple participants, generally known as conferencing,
   are more complicated.  This document defines a framework for how such
   conferencing can occur.  This framework describes the overall
   architecture, terminology, and protocol components needed for multi-
   party conferencing.

Table of Contents

   1. Introduction ....................................................2
   2. Terminology .....................................................3
   3. Overview of Conferencing Architecture ...........................6
      3.1. Usage of URIs ..............................................9
   4. Functions of the Elements ......................................10
      4.1. Focus .....................................................10
      4.2. Conference Policy Server ..................................11
      4.3. Mixers ....................................................11
      4.4. Conference Notification Service ...........................12
      4.5. Participants ..............................................13
      4.6. Conference Policy .........................................13
   5. Common Operations ..............................................13
      5.1. Creating Conferences ......................................13
      5.2. Adding Participants .......................................14

Rosenberg                    Informational                      [Page 1]
RFC 4353            Conferencing Framework with SIP        February 2006

      5.3. Removing Participants .....................................15
      5.4. Destroying Conferences ....................................15
      5.5. Obtaining Membership Information ..........................16
      5.6. Adding and Removing Media .................................16
      5.7. Conference Announcements and Recordings ...................16
   6. Physical Realization ...........................................18
      6.1. Centralized Server ........................................18
      6.2. Endpoint Server ...........................................19
      6.3. Media Server Component ....................................21
      6.4. Distributed Mixing ........................................22
      6.5. Cascaded Mixers ...........................................24
   7. Security Considerations ........................................26
   8. Contributors ...................................................26
   9. Acknowledgements ...............................................26
   10. Informative References ........................................27

1.  Introduction

   The Session Initiation Protocol (SIP) [1] supports the initiation,
   modification, and termination of media sessions between user agents.
   These sessions are managed by SIP dialogs, which represent a SIP
   relationship between a pair of user agents.  Because dialogs are
   between pairs of user agents, SIP's usage for two-party
   communications (such as a phone call), is obvious.  Communications
   sessions with multiple participants, however, are more complicated.
   SIP can support many models of multi-party communications.  One,
   referred to as loosely coupled conferences, makes use of multicast
   media groups.  In the loosely coupled model, there is no signaling
   relationship between participants in the conference.  There is no
   central point of control or conference server.  Participation is
   gradually learned through control information that is passed as part
   of the conference (using the Real Time Control Protocol (RTCP) [2],
   for example).  Loosely coupled conferences are easily supported in
   SIP by using multicast addresses within its session descriptions.

   In another model, referred to as fully distributed multiparty
   conferencing, each participant maintains a signaling relationship

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